1. Field of the Invention
This invention is directed to rendering hydrophobic filters hydrophilic. This is accomplished by coating the substrate (membrane pores) with a chemical. It has been found the chemical family of this invention will render all commercially known hydrophobic membranes hydrophilic. This chemical treatment is especially suited for polypropylene (PP) and fluorinated ethylene type polymers (fetp) which may include polytetrafluoroethylene (PTFE). It is to be especially noted that this coating renders the treated filter useful for medical device purposes and the such treated material non-toxic.
2. Description of the Prior Art
Many patents and papers (publications) have been directed toward and to making a hydrophobic filter to become hydrophilic. Among these patents, the references and citations are U.S. Pat. No. 3,853,601 as issued to TASKIER on Dec. 10, 1974; U.S. Pat. No. 4,359,510 also to TASKIER as issued Nov. 16, 1982 and U.S. Pat. No. 4,346,142 as issued to LAZEAR on Aug. 24, 1982. New developments in filter technology and in particular in small particle filters usually produces hydrophobic filters whereas it is desired that these filters are required to pass fluids.
This invention is particularly directed to polypropylene (PP) and polytetrafluoroethylene (PTFE) or membranes of like materials which are by nature hydrophobic (water repelling). Such a membrane material may be made hydrophilic (water loving) by wetting the surface of this membrane by using a liquid whose surface tension is less than the critical surface tension of the polymer. This is about thirty dynes per cm. for PP, PTFE and like type of polymers. Such treating materials include low molecular weight alcohols and solvents such as Freon (chlorinated hydrocarbon) whose surface tensions are less than the critical surface tension of PP; PTFE and like type of polymers.
Another method or process of making hydrophobic material especially polypropylene hydrophilic is subjecting the PP membrane material to a corona discharge treatment. Unfortunately, this process or method affects only the outer visible surface of the material and makes such surfaces hydrophilic. The resulting product is only useful for filters having large pore sizes (that in which the material's membrane is greater than three to five microns in size). Smaller pore size membranes for which corona discharge has been tried have produced unsatisfactory results and the membrane or filter so treated is only partially hydrophilic.
Various and many surfactant-type chemicals have been tried and have been successful to some degree with polypropylene (PP). Much poorer results have been obtained using PTFE. One problem has been unevenness of coating in which the sufactant either migrates or accumulates in one section of the membrane material making that area hydrophilic while other areas continue to exhibit hydrophilic properties and pass gases but not liquids. The resulting coated membrane may or may not pass some quantities of liquids, but the desired flow is not achieved in comparison to that obtained if a total membrane "wet out" were accomplished. More importantly, the membrane will pass gases, which is highly undesirable in many applications. Again, results when PTFE materials have been tried have been most discouraging with almost no success.
Another problem found in using known surfactant chemicals is downstream foaming which may occur as a fluid initially enters and passes through the micropores of the hydrophilic membrane and flushes out the applied coating. In use the practitioner may find this foaming undesirable because of appearance and/or as an indicator that a high concentration of the chemical is present and this chemical may be harmful. This can be avoided by adding anti-foam agents. However, this increases the total concentration of additives and generally causes increased potential toxicity problems.
Among other problems with known surfactants chemicals is that caused by the used surfactant having a "cloud point". This is the temperature at which the solution containing the surfactant becomes cloudy. This appearance change seems to occur due to the formation of a sol (gels) within the solution. This "cloud point" is defined as a definite temperature at which this phenomenon takes place for a given concentration. In reduction to practice or use this appears to actually occur over a wide temperature range. In other words, gels are present in the solution over a wide temperature range. Their concentration increases until they become highly visible as a haze in the solution at the "cloud point" temperature. This property is highly undesirable, especially when pharmaceutical type products are being prepared or filtered since the gels may form and remain in the solution on the downstream portion of the filter membrane, that is after passing through the filter. The commercially known and used chemical surfactants used for medical device filters such as Triton, Klucel and Pluronics all exhibit this phenomenon. Although the exact effect of these gels on the human circulatory system is not fully understood they are undesirable and many firms in the medical supply field are presently concerned about using products where these chemical surfactants are present.
Corona discharge treatment appears to only render the outer surface of the polypropylene filter membrane hydrophilic and unless large pore size membranes (greater than three microns) are used the corona effect does not extend to the inner filter surfaces. In addition, when corona discharge has taken effect the change of surface properties allows drug binding to occur. Similar results have been noted with PTFE membranes.
It is therefore highly desirable to maintain the existing (inert characteristics) surface properties of hydrophobic membrane filters and yet render these filters hydrophilic such that they can be used for fluid filtration particularly for pharmaceutical processes.
Another property, especially exhibited by polypropylene membranes is their ability to retain significant quantities of endotoxins. This property is highly desirable since bacteria are often killed on the surface of the membrane due to the introduction of antibiotic drugs during use. Decomposition of the bacteria present begins almost immediately in the presence of the drug and quantities of endotoxin begin to form. Under these conditions the replacement of this system is essential (usually within hours) since there is a danger that these substances will flow downstream from the filter and enter the bloodstream of the patient and cause severe toxicity problems.
A problem associated with existing hydrophilic membrane used in pharmaceutical applications is that of drug binding. A quantity of drug is held on the filter. As a result the patient receives less than the quantity of drug the physician has prescribed and may cause patient complications. Hydrophilic membranes once "wet out" exhibit little or no tendency to bind drugs. As a result, it is generally desired to maintain the inert properties of a hydrophobic membrane surface that does not chemically or physically alter the surface characteristic of the membrane while concurrently making it behave as a hydrophilic filter.
A number of surfactants including those above noted may "wet out" some of the hydrophobic membrane. Heretofore the commercially known and used chemical systems are either toxic; form gels (cloud point), require large quantities to "wet out", or alter the surface of the hydrophobic membrane such that drug binding and/or ability of the membrane to retain endotoxins is lost. For the above reasons it is very desirable to disclose a chemical system that will "wet out" hydrophobic membranes without altering their surface properties. The following disclosure employs LAS (linear alkylbenzene sulfonate) as a chemical since it is non-toxic; has no cloud point; requires small quantity for treatment, and produces no change in the hydrophobic membranes surface properties and renders polypropylene and PTFE material systems hydrophilic.